Method for gain control of field-effect transistor

ABSTRACT

The gain of a field-effect transistor having a single insulated gate is controlled by varying the forward current between the base and the source. Amplitude modulation is obtained by applying an AF signal together with a forward bias to a terminal connected to the base through a resistor. An RF signal is applied to the gate, for example through a transformer, and the amplified signal appears at a load coupled to a resonant circuit connected to the drain and one terminal of a resistor which is decoupled by a capacitor. The other terminal of the resistor is connected to a power source.

United States Patent Hayashi et al.

METHOD FOR GAIN CONTROL OF FIELD-EFFECT TRANSISTOR Inventors: YutakaHayashi; Yasuo Tarui, both of Tokyo, Japan Assignee: Kogyo Gyutsuin(also known as Agency of Industrial Science and Technology, Ministry ofInternational Trade and Industry). Tokyo-To. Japan Filed: June 21, 1972Appl. No.: 264,965

Related US. Application Data Continuationin-part of Ser. No. 24,166,March 3|, 1970. abandoned.

Foreign Application Priority Data [451 Apr. 1, 1975 [56] ReferencesCited UNITED STATES PATENTS 3,290,613 l2/l966 Theriautt 307/25l X331L756 3/1967 Nagata et al 330/35 X 3,39l,354 7/1968 Ohashi etal.......... 307/25l X 3,5l3,405 5/1970 Carlson 330/29 PrimaryE.taminerAlfred L. Brody Attorney. Agent, or Firm Robert E. Burns.Emmanuel J. Lobato. Bruce L. Adams [57] ABSTRACT The gain of afieldeffect transistor having a single insulated gate is controlled byvarying the forward current between the base and the source. Amplitudemodulation is obtained by applying an AF signal together with a forwardbias to a terminal connected to the base through a resistor. An RFsignal is applied to the gate, for example through a transformer, andthe Nov. 24 [969 Japan 44-93595 amplified Signal appears at a loadcoupled to a reso nant circuit connected to the drain and one terminal(3| 332/31 307/251 307/305" of a resistor which is decoupled by acapacitor. The 330/29 330/35 other terminal of the resistor is connectedto a power Int. Cl H03c 1/36 source Field of Search 331/3! T", 330/29,35;

307/25l, 304 8 Claims, 11 Drawing Figures 30 I-l: Z6

LOAD

f i B vi/v G CONTROL SIGN/4L. 5011 (E Ski-LU 1 HF 24 6 BKJIZMIG N) FIG.3

magma 1197s 3,875.536

Sam 2 BF 3 m't mggma H375 3.875.536

sxamaqga Reduced Gain Output Signal (No Distortion) Input Signal FIG. 5

1 Amplified and Rectified n Signal}! Reference Level METHOD FOR GAINCONTROL OF FIELD-EF F ECT TRANSISTOR This is a continuation-in-partapplication of our earlier U.S. Pat. application Ser. no. 24,166, filedMar. 31, 1970, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a method for gaincontrol of a field-effect transistor, more particularly, to a method forgain control which provided for a minimum of distortion due to crossmodulation in a field-effect transistor which does not have a tetrodeconfiguration (i.e., does not have two insulated gates).

Methods employed heretofore for the gain control,

amplitude modulation, etc., of field-effect transistors an input signaldrives the gate from the turn-on region to the cut-off region if thesignal voltage is high when the gain of the field-effect transistor isdecreased. This means that the device operates under a time variabletransfer transconductance. The resultant increased distortion due tocross modulation renders the methods virtually impracticable. An attemptto circumvent this undesirable effect has been made by the use oftetrode field-effect transistors having two insulated gate electrodes,which, however, have also given rise to various difficulties in theiroperation at high frequencies.

On the other hand, conventional efforts to make use of field-effecttransistors in the high frequency band by shortening their channellength have materialized, to give one example, in the development of amethod wherein the channel length is determined in accordance with adifference in the lengths over which two impurities are diffused. Yet,at the present, this method is also not free from drawbacks in that theproduction of such tetrode field-effect transistors according to themethod requires co'rnplex proceses.

SUMMARY OF THE INVENTION Therefore, it is a principal object of theinvention to provide an improved method for gain control ofa fieldeffecttransistor wherein all deficiencies attendant to the prior methodsmentioned above are overcome.

It is another object of the invention to provide an improved method forgain control of an ordinary fieldeffect transistor having only oneinsulated gate with a minimum of distortion due to cross modulation.

It is a further object of the invention to provide an improved methodfor gain control of an ordinary fieldeffect transistor having only oneinsulated gate, which does not need an extra bias of reverse polarity,that is, with a gain control voltage of the same polarity as that of thedrain bias voltage.

It is another object of the invention to provide an improved andexcellent method for gain control ofa fieldeffect transistor using verysimple circuits.

Characteristic features and functions ofthe invention will be furtherdescribed in connection with the accompanying drawings, in which thesame or equivalent members are indicated by the same numerals andcharacters.

BRIEF DESCRIPTION OF THE DRAWING FIG. I shows an equivalent circuit of ashort channel field-effect transistor with means for effecting gaincontrol in accordance with the invention;

FIG. 2 graphically represents the output characteristics of thefield-effect transistor due to base current;

FIG. 3 graphically represents a characteristic example of variation ingm due to a forward bias applied between a current limiting resistor ofthe base and the source;

FIG. 4 is a schematic diagram of a circuit for providing a controlledgain by the method of this invention;

FIG. 5 is a diagram illustrating control of amplification of afield-effect transistor in accordance with the invention, and

FIGS. 6 to 11 illustrate schematically various gain control circuitsthat can be used in the field-effect transistor circuit of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION In a field-effect transistor witha sufficiently short channel, as illustrated in FIG. 1, a currentbetween the drain D, and source S, electrodes increases as the deviceacts as a bipolar transistor when the base 8, is forward-biased withrespect to the source by means of a gain control 10 and when a basecurrent flows due to such forward bias. Actual examples of the abovephenomenon are schematically shown in FIG. 2, wherein the curve (a)represents an output characteristic between drain and source when thebase is kept forwardbiased by the gain control 10 up to the instant atwhich current begins to flow, whereas the curves (b), (c) and (d)respectively denote the output characteristics when the forward currentis increased.

When utilized as a tuned amplifier, the field-effect transistor has itsdrain connected with an LC resonance circuit 12; in this case, the pointof operation undergoes a series of changes as indicated by A, B, C and Din FIG. 2 along with increases in the base current after furtherconnecting a resistance '14 with a bypass capacitor 16 in series withthe LC resonance circuit 12. Accordingly, the drain voltage graduallydecreases. On the other hand, since the value gm relative to thefieldeffect transistor itself decreases substantially in proportion to adecrease in the drain voltage when the drain voltage is less than apinch-off voltage, the signal am plification factor measured across aload coupled to the resonant circuit decreases along with decrease in gmand, further increase in the output conductance.

In the above instance, the gate bias voltage applied by a gate biascircuit 18 is not changed, and the bias voltage between base and sourceelectrodes due to the gain control 10 is also kept almost unvaried, sothat the change in the threshold voltage of the field-effect transistoris only negligible and the effective gate voltage, obtained bysubtracting the threshold voltage from the gate voltage, is kept nearlyat a constant level. It is accordingly possible to decrease only thegain without any corresponding increase in distortion due to crossmodulation even though the amplitude of the signal applied to the gateG, by an input signal source device 20 may be large.

Shown in FIG. 3 is an example of a characteristic curve indicated by thevalue gm with regard to the forward voltage present between base andsource when a high resistance is connected in series with the base.

Further, at frequencies in excess of the upper limit of frequenciesacceptable in a bipolar transistor, without varying the drain voltage,the output conductance increases due to a charge injected to the base,so that the desired gain control is made possible by the base forwardvoltage or current. For the purpose of amplitude modulation, a carrierwave may be applied to the insulated gate (indicated by G, in FIG. 1),and a signal of a frequency lower than that of the carrier wave may beapplied to the base terminal.

in the circuit shown in FIG. 4, in which the method of this invention isapplied, an n-channel MOS transistor capable of operating in bipolarfashion is indicated at 40. The gate, drain, base and source electrodesare indicated at G, D, B, and S respectively. An input RF signal from asignal source 22 is passed to the gate electrode G by means of atransformer 23, the secondary of which forms part of a resonant circuitwith a variable capacitor 24. The amplified signal appears at theresonant circuit 25 connected to the drain, and is passed through atransformer 26, the primary of which forms part of the resonant circuit25, to a load represented by the resistor 27. A bias voltage V decoupledby a capacitor 28 is applid to the gate electrode as shown. A positivesupply voltage V decoupled by a resistor 29 having resistance R andcapacitor 30 is applied through resonant circuit 25 to the drainelectrode. The gain control signal represented by a controlled positivevoltage supplied by the gain control means 31, is applied through aresistor 32 to the base electrode.

The gain control obtained with the circuit of FIG. 4 is illustrated byFIG. 5 in which curves M and N represent I vs. V with different gains.it will be seen that the curve is merely changed in inclination so thatvariation in gain is obtained without distortion.

Amplitude modulation can be obtained with the circuit by applying an AFsignal together with a positive bias to the base B by means of thecircuit 31.

The gain control circuit which is indicated by the block in FIG. 1 is acircuit which can supply variable and controllable forward currentbetween the base and source of the field-effect transistor (FET). Thus,it can be either a variable voltage source with a resistor in series asillustrated by way of example in FIG. 6 or a variable current source asillustrated in FIG. 7. The value of the voltage or current is controlledby hand in the case of manual control by a control signal in the case ofautomatic gain control (AGC) or by an AF signal in the case of amplitudemodulation. Simple and practical ways of providing controlled baseforward current are illustrated in FIGS. 8 and 9 where the gain of theamplifier is manually controlled by adjusting a potentiometer R,.. Acircuit for control by amplitude modulation is shown in FIG. 10 where DCbias current can be adjusted by a potentiometer R, and modulationefficiency can be determined by a resistance R, Automatic gain control(AGC) can be achieved by a circuit as illustrated in FIG. 11 where anamplified and rectified rfsignal is compared with a reference level andthe difference between them is amplified by a differential amplifier A.The output voltage from the amplifier A is the control signal. if theamplifier A has a low output resistance the output is supplied to thebase terminal through a series resistor R,. If the amplifier A is ofhigh output resistance or current source type, the output can bedirectly supplied to the base terminal of the FET.

The control circuits illustrated in FIGS. 6 to 11 are also applicable tothe field-effect transistor circuit illustrated in FIG. 4 where the gaincontrol is represented by the block 31. In this event the seriesresistor R, is the resistor 32 shown in H0. 4.

As herein used the term forward bias means a positive bias for thep-type base of an n-channel lGFET with reference to the source and anegative bias for the n-type base of a p channel lGFET with reference tothe source. The polarity shown in all figures of the drawings is for ann-channel FET. The polarity must be re versed for a p channel FET.

While preferred embodiments of the invention have been shown by way ofexample in the drawings, it will be understood that the invention is inno way limited to these embodiments.

What we claim and desire to secure by letters patent l. A method forgain control and amplitude modulation of a field effect transistordevice provided with at least a gate electrode, a gate insulator, asource, a base and a drain, which device is capable of performing as abipolar transistor when said source, base and drain are used as anemitter, a base and a collector respectively, comprising the steps ofproviding a resistor, a resonant circuit and a power source in serieswith the source-drain circuit, capacitively decoupling the resistor,coupling a load to said resonant circuit, applying a bias voltage tosaid gate electrode, applying a first signal to said gate electrode,applying a second signal between said base and source electrodescomprising a forward base-source current, and increasing and decreasingthe forward base-source current by altering said second signal toprovide gain control and amplitude modulation.

2. A field effect transistor circuit comprising a field effecttransistor having a gate electrode, a gate insulator, a source, a baseand a drain, and being capable of operating as a bipolar transistor whensaid source, base and drain are used as an emitter, a base and acollector, respectively, a resistor, a resonant circuit, a load coupledto said resonant circuit and a power source connected in series with thesource-drain circuit, capacitor means decoupling said resistor, a biasvoltage source connected to said gate electrode, a first signal sourcecoupled with said gate electrode, a second signal source comprising aforward base-source current connected between said base and sourceelectrodes and including means for selectively increasing and decreasingthe forward base-source current to provide gain control and amplitudemodulation.

3. A circuit according to claim 2 in which said source-drain circuitcomprises an LC resonant circuit connected to said drain electrode inseries with said resister.

4. A circuit according to claim 3, in which said LC resonant circuitincludes the primary of a transformer, the secondary of which isconnected to a load resistance.

5. A circuit according to claim 4, in which said decoupling meanscomprises a capacitor connected to said source-drain circuit betweensaid LC resonant circuit and said resistor.

6. A circuit according to claim 2. in which said first signal source isconnected with the primary of a trans- 8. A circuit according to claim6, in which said source is connected to ground and a capacitor has oneterminal connected to ground and another terminal connected between saidbias voltage source and the secondary of said transformer.

=1 II i

1. A method for gain control and amplitude modulation of a field effecttransistor device provided with at least a gate electrode, a gateinsulator, a source, a base and a drain, which device is capable ofperforming as a bipolar transistor when said source, base and drain areused as an emitter, a base and a collector respectively, comprising thesteps of providing a resistor, a resonant circuit and a power source inseries with the sourcedrain circuit, capacitively decoupling theresistor, coupling a load to said resonant circuit, applying a biasvoltage to said gate electrode, applying a first signal to said gateelectrode, applying a second signal between said base and sourceelectrodes comprising a forward base-source current, and increasing anddecreasing the forward base-source current by altering said secondsignal to provide gain control and amplitude modulation.
 2. A fieldeffect transistor circuit comprising a field effect transistor having agate electrode, a gate insulator, a source, a base and a drain, andbeing capable of operating as a bipolar transistor when said source,base and drain are used as an emitter, a base and a collector,respectively, a resistor, a resonant circuit, a load coupled to saidresonant circuit and a power source connected in series with thesource-drain circuit, capacitor means decoupling said resistor, a biasvoltage source connected to said gate electrode, a first signal sourcecoupled with said gate electrode, a second signal source comprising aforward base-source current connected between said base and sourceelectrodes and including means for selectively increasing and decreasingthe forward base-source current to provide gain control and amplitudemodulation.
 3. A circuit according to claim 2 in which said source-draincircuit comprises an LC resonant circuit connected to said drainelectrode in series with said resistor.
 4. A circuit according to claim3, in which said LC resonant circuit includes the primary of atransformer, the secondary of which is connected to a load resistance.5. A circuit according to claim 4, in which said decoupling meanscomprises a capacitor connected to said source-drain circuit betweensaid LC resonant circuit and said resistor.
 6. A circuit according toclaim 2, in which said first signal source is connected with the primaryof a transformer, the secondary of which is connected between said biasvoltage source and said gate electrode.
 7. A circuit according to claim6, in which said source is connected to ground and in which a variablecapacitor has one terminal connected to ground and another terminalconnected to said gate electrode.
 8. A circuit according to claim 6, inwhich said source is connected to ground and a capacitor has oneterminal connected to ground and another terminal connected between saidbias voltage source and the secondary of said transformer.